Reuter , Runa Turid

Rapid Arctic warming is driving extensive sea-ice loss and transforming thick multiyear ice into thinner, more fragile first-year ice. The resulting changes in light availability, freshwater input, and upper water column stratification are fundamentally reshaping biological and biogeochemical processes, including the export of particulate organic carbon (POC). Despite acting as an important sink for atmospheric CO2, the spatial variability, magnitude, and dominating pathways of biologically mediated carbon export in the central Arctic Ocean remain poorly constrained, particularly under rapidly changing environmental conditions. To better understand the role of the rapidly changing central Arctic Ocean in biologically mediated carbon export and sequestration, this thesis investigates the magnitude, spatial variability, and dominating export mechanisms of particulate organic carbon export under predominantly first-year sea ice during summer 2023 in relation to prevailing pelagic and sea-ice conditions (Manuscript 1), the variability of carbon export across contrasting sea-ice regimes of different age and origin during the summer melt season of 2025 (Manuscript 2), and the importance of the central Arctic Ocean for biologically mediated carbon export in comparison to other Arctic and marine systems (Manuscript 3). Under-ice biogeochemical fluxes were quantified using short-term sediment trap deployments at 5, 20, 40, and 90 m depth, complemented by analyses of the size and composition of exported particles. Despite reduced nutrient concentrations in the upper water column, particularly nitrate (Manuscript 1), and low phytoplankton biomass (Manuscript 2), particulate organic carbon export fluxes at 90 m depth remained remarkably high across all sediment trap deployments (summer 2023: 71 ± 41 mg C m-2 d-1 (Manuscrip 1); summer 2025: 81 ± 25 mg C m-2 d-1 (Manuscript 2)). Carbon export fluxes were largely decoupled from prevailing sea-ice and oceanographic conditions, although the sampled sea-ice regimes differed substantially, ranging from predominantly first-year ice of central Arctic Ocean origin during summer 2023 (Manuscript 1) to first-year ice originating from the Kara Sea, second- and multiyear ice of Siberian origin, and old multiyear ice associated with the Last Ice Area north of Greenland and Canada during summer 2025 (Manuscript 2). In addition, sea-ice thickness and upper-ocean conditions changed substantially over the course of the 2025 melt season (Manuscript 2). Rather than being characterized by short-lived pulses of enhanced carbon export driven by the rapid release and sedimentation of ice-associated algae during sea-ice melt, the central Arctic Ocean export regime during the summers 2023 and 2025 was sustained and predominantly mediated by pelagic processes and the transformation of organic matter into fast-sinking fecal pellets by zooplankton (Manuscript 1 and Manuscript 2). Particularly during summer 2025, copepod and appendicularian fecal pellets accounted for a major fraction of the exported carbon (Manuscript 2). 18S rRNA gene sequencing of appendicularian fecal pellets, copepod fecal pellets, and appendicularian houses collected during summer 2025 below the Chlorophyll a maximum suggested an efficient trophic pathway in which microbially and picoplankton-derived carbon is first processed by appendicularians and subsequently transferred to copepods via consumption of appendicularian fecal pellets. Synthesis of short- and long-term sediment trap observations across the central Arctic Ocean indicates that the region can sustain efficient particulate organic carbon fluxes during the summer months, with magnitudes overlapping those observed in productive Arctic and lower-latitude systems, including the Cape Blanc upwelling region (Manuscript 3). Annually integrated estimates derived from the limited available sediment trap observations suggest that the (central) Arctic Ocean plays a disproportionately important role in global biologically mediated carbon export relative to its small surface area. Sea-ice melt is expected to intensify in the future central Arctic Ocean, strengthening upper-ocean stratification and reducing the supply of nutrients from deeper water masses. In such environments, where the phytoplankton community is projected to shift towards smaller-sized taxa and ice algae may become less abundant, alternative carbon export pathways that are less directly dependent on the magnitude of primary production, as observed in this thesis, may become increasingly important for sustaining an efficient biological carbon pump in the (central) Arctic Ocean.

Reuter , Runa Turid

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